Titanium dioxide (TiO2) is oftentimes used as a coating for a variety of articles, for example, glass-containing work pieces such as, but not limited to, glass sheets or a continuous glass float ribbon; metal or metal-containing work pieces such as lens or mirrors; ceramic work pieces; and other materials, due to its high refractive index, high dielectric constant, and optical transmittance particularly in the visible and near-AIR region. These coated articles are used in a variety of industries such as, but not limited to, construction, optics, aerospace, magnetic memory manufacturing, and automobile.
One particular application for titanium dioxide (TiO2) coatings is to provide a photocatalytically-activated self-cleaning (PASC) surface on an article such as glass windows used in the construction industry. These coatings, when applied to the surface of the glass window that is exposed to environmental conditions, render the surface superhydrophilic after activation, i.e., exposure to UV radiation. An activated PASC coated surface can have a water contact angle as low as 10 degrees compared to water contact angles of up to 40 degrees on untreated surfaces. At such low contact angles, water readily spreads out into a film, and promotes the removal of environmental residues, such as dust and dirt, from the window surface. Because of this, PASC coated glass is also referred to as self-cleaning glass since water alone is capable of lifting and removing foreign materials from its surface.
The TiO2 coating can be deposited onto an article using a variety of deposition methods, including chemical vapor deposition (CVD) such as atmospheric pressure chemical vapor deposition (APCVD), spray pyrolysis, and vacuum deposition methods such as magnetron sputtered vacuum deposition (MSVD). In a typical CVD process, gaseous or vapor phase metal precursors are carried to the heated surface of the article by a flow of gas. The reagents react on the surface to form a thin coating. In vacuum deposition, one or more articles to be coated are passed into an evacuated chamber. A plasma (glow discharge) is formed from a plasma-producing gas such as argon, helium, etc., introduced into the evacuated system. The ions in the plasma are accelerated at a cathode “target” made from the substance that is to be deposited onto the article surface. The impact of the plasma ion dislodges material from the target. The dislodged material deposits on the surface, forming a thin coating. In reactive sputtering, a reactive gas chemically combines with the dislodged material from the target to form a new chemical species that deposits on the surface. For example, a TiO2 coating is deposited onto the article surface using a titanium-containing target in a reactive oxygen atmosphere.
These processes not only deposit TiO2 on the surface of the article but also deposit TiO2 unproductively upon the inner walls and other surfaces within the reactor chamber in which the processes are being performed. After a certain number of process cycles, the TiO2 coating on the inner surfaces within the reactor can grow to such a thickness that may result in flaking of the residues from the walls. These residues can find their way onto the article being coated and form defects in the coating. In certain applications such as when the article to be coated is a glass work piece, defects can occur such as blemishes within the glass, poor adhesion of the TiO2 coating to the article surface, and other abnormalities. Such defects can result in an unacceptable product and/or loss of yield. Further, in certain deposition methods, such as sputtering, the surface residues can interfere with the sputtering process thereby decreasing product yield.
Deposition reactors are cleaned periodically to mitigate the occurrence of defects. Current industry practice is to clean the reactor by taking the reactor offline and mechanically removing (scraping, abrading, brushing and/or shot blasting) the wall coating. Yet other methods involve taking the reactor off-line and using wet chemicals alone or with the aforementioned mechanical methods to remove the deposition residues from the internal surfaces of the reactors and any fixtures contained therein. Still another method in cleaning the reactor involves coating the entire chamber with a sacrificial film such as aluminum foil and then disposing of the foil after one or more production cycles. All of these methods typically result in costly reactor down time before and/or after the cleaning operation is conducted.
Other cleaning methods may be conducted during the production cycle. In CVD reactors, for example, the reactor walls are typically sprayed with nitrogen and scraped with special tools that withstand the high reactor temperature. Since the production line cannot be stopped during the cleaning operation, the scraped debris falls onto the surface of the article and produces unwanted defects on the surface. This unacceptable product must be re-melted or thrown away. After a moderate number of cycles, manual cleaning is no longer effective and the reactor must be taken completely out of the process, cooled and rebuilt. In sputter deposition reactors, the reactor must be taken completely offline, pressurized to ambient, opened up and mechanically cleaned. Although the actual mechanical cleaning does not take a large amount of time or labor, reassembling and evacuating the sputter reactor may take several hours.
Clearly, there is a need for a process to efficiently and effectively chemically dry clean the TiO2 residues from the surfaces within a reactor. An effective chemical dry cleaning process can significantly increase the productivity and lower the cost-of-ownership (CoO) of the TiO2 deposition process.